The present invention relates generally to printing an image and more particularly to printing an image with N-tones.
Numerous methods are available to print a halftone image from a grey scale image. The methods usually involve establishing the approximate grey level of each pixel of the grey scale image, and then, based on some representation schemes, printing dots to represent the grey scale image.
One form of representation scheme depends on a dither matrix, which has the same number of pixels as the grey scale image. Each pixel in the matrix has a level, which is compared to the level of its corresponding pixel in the grey scale image to produce the level of a pixel in the halftone image. A general discussion of a dither matrix to render an image can be found in "Digital Halftoning," by R. Ulichney (1987). Another representation scheme is known as the error diffusion technique, with a general discussion found in "An Adaptive Algorithm for Spatial Greyscale," written by Floyd and Steinberg, and published in the Proc. SID, Volume 17, pages 75-77, 1976.
In printing the halftone image, it is preferred to have inconspicuous dots in the image. There is a constant need to generate an image with imperceptible dots.
One method to generate such a visually pleasing image is to significantly increase the resolution of the halftone image to a high number of dots-per-inch, such as 2400. The machine implementing such a high resolution printing is usually very expensive because the particle to generate each dot has to be very small, which can be difficult for normal dry toner particles of laser printers. Moreover, each dot has to be positioned very accurately onto the desired location.
It should be apparent from the foregoing that there is still a need for a method to print an image with imperceptible dots in an inexpensive manner.